The science behind the song stuck in your head. (Yes, old link for those who visit the ttlg forums, but I only got around to posting it now.)

"The first case of Stuck Tune Syndrome is lost to history. If ancient Romans had "Parvus Orbis Est" (Latin for "It's a Small World") chirping incessantly in their heads, they were kind enough not to mention it."

Hehe. Oh yes, thought I should mention. It's perfectly possible to get classical music stuck in your head. I should know, it happens to me all the time. --; Once I even had the whole first act of La Traviata stuck in my head. ^^;;;